In my experience, there are two factors that need to be dealt with on IWB-style belt clip holsters.
First is the location of the belt clip. When placed over the bulk of the weapon (especially revolver cylinders) this adds bulk to the package, and also projects out where the clip can snag on things repeatedly.
Second is the belt clip itself. As Sipowicz has noted, the clip has to be a good fit to the belt used. I have also found that the clips generally available from suppliers are of varying quality, frequently lacking the strength to grip securely, and frequently being too flimsy for long-term service.
Earlier this year I started a project for an offset belt clip IWB holster and made up a few for testing (Sipowicz was one of my testers, and the holster for his HK P2000 is one of the prototypes). One of the results of that testing was identifying the need for a durable and sturdy belt clip, so I contacted manufacturers and found one that agreed to produce what I wanted. I have an order pending production, which is taking longer than we anticipated, and I hope to have my first shipment within the next couple of weeks. At that time I will be able to start filling orders, and I will add this model to my website.
I do recommend that a belt be used with this type of holster. The clips that I am having made are very strong, have a profile intended to clamp securely around the belt, and I think they will grip firmly on a trouser waistband, but there will always be the possibility of a closely-fitted holster coming out with the handgun when drawn, especially in a tense situation. This possibility can be greatly reduced by giving the clip a belt to clench onto.
I find this style of holster to be very convenient for slipping a holstered handgun into the waistband easily, and easily removing it. Those with the need to go into and out of areas where weapons are not allowed or not advisable can make use of this type of holster. While not a holster design that I would choose for extended wear, for known dangerous situations, or for range use, I think this style fills a need for many people and circumstances.
As with any holster design, there are always compromises among the factors of comfort, accessibility, security, and concealability. There is no single "perfect" holster design for every user or every situation.